In recent years, an amount of communication of information has been continuously increasing. For example, in devices such as an IC card and a mobile phone terminal, communication is frequently performed in a high-frequency region such as microwave or millimeter wave. Therefore, there has been required a printed wiring board allowing for small transmission loss when used in such a high-frequency region, i.e., there has been required a printed wiring board excellent in high-frequency characteristic. As a substrate for manufacturing such a high-frequency printed wiring board, a substrate in which a copper foil is layered on a dielectric layer has been generally used.
As a technique for improving the high-frequency characteristic of the printed wiring board, it has been considered to use, as a material of the dielectric layer, a composite material in which an inorganic filler such as minute glass or a particulate ceramic filler material is included in a fluororesin matrix such as polytetrafluoroethylene (see U.S. Pat. Nos. 4,886,699 and 5,358,775). Moreover, as a method for forming the dielectric layer using such a composite material, the following method has been proposed: dipropylene glycol is mixed as a lubricant to the fine fluoropolymer powder and the inorganic filler, and cold extrusion into a form of a sheet is performed.
Meanwhile, since the fluororesin has a small surface energy, an adhesive strength between the dielectric layer and the copper foil is small. As a technique for increasing the adhesive strength between the dielectric layer including the fluororesin and the copper foil, the following method has been considered: a silane-based coupling agent having a functional group including a N atom or S atom is present in the vicinity of an interface therebetween (see WO 2014/192718).